As is generally known, a torque converter has merits including a torque-fluctuations absorption function owing to power transmission between input and output elements via fluid and a torque-multiplication function, and demerits including a loss of torque, in other words, a reduced torque transmission efficiency. For the reasons discussed above, recently, automotive vehicles often use lock-up torque converters in which input and output elements (pump and turbine elements) can be directly coupled with each other by a lockup clutch under engine/vehicle operating conditions that do not require a torque-fluctuations absorption function and a torque-multiplication function to eliminate internal slippage and improve fuel economy, or a slip-rotation speed of the lockup torque converter, that is, a speed difference between the input and output elements can be controlled or adjusted properly by way of slip lockup control for the lockup clutch depending on engine/vehicle operating conditions.
One such slip lockup control system for a transmission torque-converter lockup clutch has been disclosed in Japanese Patent Provisional Publication No. 2000-145948 (hereinafter is referred to as “JP2000-145948”). In the torque-converter slip control system of JP2000-145948, a target slip-rotation speed is calculated or estimated based on engine/vehicle operating conditions, and also an actual slip-rotation speed corresponding to the rotation-speed difference between input and output elements is detected and calculated. The target slip-rotation speed is compensated for by means of a pre-compensator to produce a target slip-rotation speed correction value. The torque-converter slip control system performs a feedback control based on the deviation between the actual slip-rotation speed and the target slip-rotation speed correction value, to feedback-control a capacity of engagement of the lockup clutch so that the actual slip-rotation speed is brought closer to the target slip-rotation speed.